The present invention relates to the preparation of highly stable, high surface area catalyst carrier materials derived from hydrotalcite-type materials by calcination at an elevated temperature.
The dehydrogenation of paraffins to olefins is of considerable commercial importance due to the need for olefins for the manufacture of products such as high octane gasolines, synthetic elastomers, detergents, plastics, ion exchange resins and pharmaceutical products. For a dehydrogenation process to be commercially useful, it must utilize catalysts exhibiting a high activity, a high rate of conversion, a high selectivity for the formation of olefins, and a high stability.
A large number of catalysts are previously known for the dehydrogenation of paraffins. These catalysts comprise a solid carrier material on an inorganic oxide base and various catalytic metals and promoter metals deposited on the carrier material or incorporated into the carrier material by other means. Carrier materials on an alumina base have been widely used in such dehydrogenation catalysts.
U.S. Pat. No. 4,788,371 discloses such catalyst and a process for the steam dehydrogenation of dehydrogenatable hydrocarbons with oxidative reheating. A dehydrogenatable C2-30 hydrocarbon, steam and an oxygen-containing gas are contacted in a reaction zone with a catalyst comprising a Group VIII noble metal, one or more components selected from lithium, potassium, rubidium, cesium and francium, and a component selected from boron, gallium, indium, germanium, tin and lead, deposited on an inorganic oxide carrier material. The preferred carrier material is alumina having a surface area of 1-500 m2/g, preferably 5-120 m2/g. Alumina is employed as the catalyst carrier in all the working examples of the patent. A preferred catalyst according to said U.S. patent contains about 0.70 wt. % of platinum, about 0.50 wt. % of tin and about 3.86 wt. % of cesium, and has a surface area of about 85 m2/g.
Mixtures of magnesium oxide MgO and alumina Al2O3 and mixed oxides of Mg and Al have also been utilized as catalysts, and as carrier materials for catalysts. International Patent Application No. PCT/JP89/00053 discloses an alkoxylation catalyst comprising a magnesium oxide that has been modified by adding thereto at least one trivalent metal ion, preferably selected from Al3+ and Ga3+. British Patent Application GB 2,225,731 discloses a catalyst for hydrotreatment, e.g. hydrodemetallization or hydrodesulphurization, comprising in a substantially homogenous phase magnesia and alumina wherein the molar ratio of Mg to Al is preferably from 3:1 to 10:1, together with a Group VI metal and/or at least one Group VIII metal.
Hydrotalcite is a layered mineral of formula: Mg6Al2(OH)16CO34H2O. Over the years, a large number of hydrotalcite-like compounds, of general formula: [M(II)1xe2x88x92xM(III)x(OH)2]x+(Anxe2x88x92x/n) mH2O, where A=anions, have been prepared. Cavani, F. et al., Cat. Today, vol.11, no.2, 173 (1991). These compounds are characterized by a sheet-like structure, in which the anions are located in the interlayer between two brucite-like sheets containing the metal ions. MII1 MIII metal ions having an ionic radius which is not too different from Mg2+ can form hydrotalcite-like compounds. Cavani, F. et al., supra.
Upon calcination at 400-700xc2x0 C., a high surface area (typically 160-220 m2/g) material with an XRD pattern typical for MgO is formed, without separation of the two metal ions into separate oxide phases. Schaper, H., et al., Appl. Cat., vol. 54, 79 (1989). Upon calcination at even higher temperatures, the mixed oxide is gradually transformed into a spinel structure, i.e., MIIMIII2O41 with a much lower surface area. McKenzie, A. L., et al., J. Catal., vol. 138, 347 (1992); Bellotto, M., et al., Phys. Chem., vol. 100, 8535 (1996). One major use for these materials is as support materials for catalysts, (see, Cavani, F. et al., supra) for instance for the catalytic dehydrogenation of lower alkanes. Akporiaye, D. et al., Norwegian Patent No. 179131 (1993). It has been reported that certain materials formed by calcination of a Mgxe2x80x94Al-containing hydrotalcite at 300-700xc2x0 C. exhibit a high stability towards sintering in a humid atmosphere. See, Schaper, H., et al., Appl. Cat., supra.; Schaper, H., European Patent No. 0 251 351 (1988).
The present invention provides a catalyst which has improved catalytic performance compared to prior art catalysts with regard to catalyst activity, and at the same time exhibits an increased catalyst life time by preventing irreversible deactivation like sintering of the support.
In one early embodiment of the invention described in co-pending U.S. patent application Ser. No. 08/569,185, it had been found that if a mixed oxide of Mg and Al is used in combination with a Group VIII noble metal and certain promoters of the kind disclosed in the above-mentioned U.S. Pat. No. 4,788,371, a catalyst can be obtained which exhibits improved activity and stability when used for dehydrogenating dehydrogenatable hydrocarbons.
The carrier for that embodiment of the catalyst may be prepared by adding a solution of sodium hydroxide and sodium carbonate to a solution of magnesium nitrate and aluminum nitrate according to the method described in Journal of Catalysis, vol. 94, pp.547-557, (1985), incorporated herein by reference. Instead of sodium hydroxide and sodium carbonate, potassium hydroxide and potassium carbonate can be used, see Applied Catalysis, vol. 55, pp. 79-90 (1989), incorporated herein by reference. A hydrotalcite-like compound Mg6Al2 (OH)16CO3-4H2O is formed by evaporation (drying) of the above-mentioned mixtures. The hydrotalcite is then calcinated at a temperature 500-800xc2x0 C. to give Mg(Al)O. The molar ratio of Mg to Al typically ranges from 1:1 to 10:1, and the surface area is typically ranging from 100 to 300 m2 per gram, preferably from 140 to 210 m2 per gram, and the particle size can be in the range of 100 xcexcm to 20 mm.
The calcination temperature for that embodiment of the catalyst was within the range of about 500 to about 800xc2x0 C. A calcination temperature that had been shown to produce good results was about 700xc2x0 C. In some of the examples set forth herein, this temperature was held for about 15 hours.
It has now been found, however, that the stability of the catalyst described herein could be further improved.
Thus, the present invention provides for a catalyst support material comprising a mixed oxide consisting essentially of a divalent metal and a trivalent metal in a substantially homogeneous phase, which is a calcination product of a hydrotalcite-like phase calcinated at a temperature of about 700-1200xc2x0 C., wherein the divalent metal/trivalent metal molar ratio is equal to, or higher than 2.
Tests of the effect of the calcination temperature of hydrotalcite and hydrotalcite-like materials at different temperatures from 700xc2x0 C. to 1200xc2x0 C. were therefore investigated.
By performing these investigations, it has been surprisingly found that by raising the calcination temperature of the catalyst support precursor hydrotalcite to 700xc2x0 C. to 1200xc2x0 C., preferably to the range of 750 to 950xc2x0 C., an improvement of the catalyst stability could be achieved with an acceptable reduction in the surface of the catalyst carrier compared to the gain in stability at use. In a further aspect, the present invention thus relates to a catalyst support material comprising a mixed oxide consisting essentially of Mg and Al in a substantially homogenous phase, which is a calcination product of a hydrotalcite phase, preferably calcinated at a temperature of 750 to 950xc2x0 C., wherein the Mg/Al molar ratio is equal 2 or higher than 2. A most preferred range for the calcination has been found to be at 770 to 850xc2x0 C., and within that range, the preferred temperature is at about 800xc2x0 C.
Preferably the Mg/Al molar ratio is in the range of about 2.5 to 6.0, and most preferably, the Mg/Al molar ratio is in the range about 3 to about 5.
In another aspect of the present invention, a method for preparing said catalyst support material is provided wherein a solution comprising a divalent metal salt and trivalent metal salt is mixed with a basic aqueous solution, the reaction product recovered from said reaction mixture, said product being washed and dried, and the dried product is calcinated at a temperature ranging from about 700-1200xc2x0 C. Calcination temperatures in the range of 750-950xc2x0 C. have been found particularly suitable. More preferably the calcination takes place at a temperature ranging from about 770 to about 850xc2x0 C. The best results have so far been achieved when the calcination was performed at about 800xc2x0 C.
The calcination may be effected, for example, for about 1 to about 20 hours, and preferably the calcination is effected for about 2-15 hours.
The basic aqueous solution used in this process is preferably a composition of aqueous ammonium or alkali metal hydroxides and carbonates.
The preferred divalent metal therein is Mg and the preferred trivalent metal therein is Al.
The molar ratio of hydroxide to carbonate may, for example, be within the range of 1:1 to 3:1.
In another aspect, the present invention relates to a dehydrogenation catalyst comprising a transition metal, preferably a transition metal selected from the first row of transition metals of the Periodic Table and/or a Group VIII metal, impregnated on to the catalyst support described above.
Preferably the first row transition metal is Cr.
Preferably this catalyst comprises both a Group IVA metal and a Group VIII metal impregnated onto the catalyst support material mentioned above. Optionally a Group IA metal may be used together with the Group VIII metal and the Group IVA metal.
Preferably the Group VIII is Pt, the Group IV metal is Sn and the Group IA metal is Cs. Preferably the Group VIII metal catalyst is in the range of 0.05 to 5.0 percent by weight and the amount of the Group IVA metal is 0.05 to 7.0 percent by weight, optionally Group IA 0.05 to 5 percent by weight.
The present invention also relates to a process for the catalytic dehydrogenation of light alkanes wherein a stream of such light alkanes is passed through a layer of the catalytic active compositions described above in the presence or absence of steam.
Thus, according to one embodiment this process is performed in the presence of steam and in another embodiment, the process is performed in the absence of steam.
The present invention also relates to the use of the catalytic composition as described above for the dehydrogenation of light alkanes.
It had been found that the materials covered by the embodiment of the invention disclosed in co-pending United States patent application Ser. No. 08/569,185 and described in more detail herein, predominantly maintains the MgO structure, and also a high specific surface area and exhibited an improved stability towards sintering compared to the materials reported in Schaper, H., et al., Appl. Cat., vol. 54, 79 (1989) and Schaper, H., European Patent No. 0 251 351 (1988), supra.
Preferably, the catalyst has been subjected to a pretreatment comprising a reduction, preferably in hydrogen, a subsequent oxidation, preferably in air optionally mixed with nitrogen, and finally a second reduction, preferably in hydrogen (ROR pretreatment; ROR=Reduction-Oxidation-Reduction).
The Group VIII noble metal is preferably selected from platinum and palladium, with platinum being the most preferred. The Group IVA metal is preferably selected from tin and germanium, with the most preferred metal being tin.
It has further been shown that the selectivity of the catalysts of the invention in a dehydrogenation process is further improved by including therein a Group IA alkali metal, preferably cesium or potassium, most preferably cesium.
It is remarkable that the new catalyst exhibits a very high activity in the dehydrogenation of hydrocarbons even with a low content of Group VIII noble metal of e.g. 0.2-0.4 wt. %.
The Group VIII metal, the Group IVA metal and the optional Group IA metal can be incorporated into the carrier by any of the methods known in the art. A preferred method consists in impregnating the oxide carrier with solutions or suspensions of decomposable compounds of the metals to be incorporated.
The catalyst and its preparation are described in more detail below with reference to embodiments wherein platinum, tin and optionally cesium are deposited on the carrier material, but the description is also valid for the deposition of other metals within the scope of the invention, with any adaptations that will be obvious to a person skilled in the art.